1. Field of the Invention
The present invention relates to variants of a cellobiohydrolase II, polynucleotides encoding the variants, methods of producing the variants, and methods of using the variants.
2. Description of the Related Art
Cellulose is a polymer of the simple sugar glucose covalently linked by beta-1,4-bonds. Many microorganisms produce enzymes that hydrolyze beta-linked glucans. These enzymes include endoglucanases, cellobiohydrolases, and beta-glucosidases. Endoglucanases digest the cellulose polymer at random locations, opening it to attack by cellobiohydrolases. Cellobiohydrolases sequentially release molecules of cellobiose from the ends of the cellulose polymer. Cellobiose is a water-soluble beta-1,4-linked dimer of glucose. Beta-glucosidases hydrolyze cellobiose to glucose.
The conversion of lignocellulosic feedstocks into ethanol has the advantages of the ready availability of large amounts of feedstock, the desirability of avoiding burning or land filling the materials, and the cleanliness of the ethanol fuel. Wood, agricultural residues, herbaceous crops, and municipal solid wastes have been considered as feedstocks for ethanol production. These materials primarily consist of cellulose, hemicellulose, and lignin. Once the lignocellulose is converted to fermentable sugars, e.g., glucose, the fermentable sugars are easily fermented by yeast into ethanol.
WO 2006/074005 discloses variants of a Hypocrea jecorina cellobiohydrolase II. Heinzelman et al., 2009, Proceedings of the National Academy of Sciences USA 106:5610-5615 discloses a family of thermostable fungal cellulases created by structure-guided recombination. Heinzelman et al., 2009, Journal of Biological Chemistry 284, 26229-26233 discloses a single mutation that contributes to stability of a fungal cellulase.
It would be advantageous in the art to improve the ability of polypeptides having cellobiohydrolase activity to improve enzymatic degradation of lignocellulosic feedstocks.
The present invention provides variants of a parent cellobiohydrolase II with increased thermostability compared to its parent.